A thermal printing head is an array of contiguous thermal resistance elements. By selectively passing an electric circuit through certain of the elements while a heat sensitive paper is progressively advanced over the top of the head, facsimile printing can be achieved.
One arrangement for driving the head is to individually access each thermal resistance element. This is generally accomplished by utilizing a current sinking transistor for each resistive element. Because of the large number of interconnecting leads required, the transistor elements and the accompanying decoding logic must be mounted on the thermal head, resulting in a relatively costly and complex structure.
An alternate arrangement is to connect the array as a matrix of rows and columns. Printing is then achieved by simultaneously applying a voltage between say one of the columns and selected ones of the rows, and thereafter sequentially repeating the operation until all columns have been accessed to print one complete line. The heat sensitive paper is then advanced relative to the head after which the operation is repeated to print the following line.
This matrix arrangement contains a large number of paralleled leakage paths which under certain operating conditions can result in sufficient voltage drop across certain of the unaccessed elements to cause spurious printing. One arrangement which circumvents this utilizes a diode connected in series with each thermal printing element which blocks the applied d-c voltage from passing through the reverse leakage paths. With large arrays such as those containing over 1,000 elements, the mounting and connection of these diodes to the thermal elements requires a large number of bonds which increase costs and reduce reliability.
In an alternate arrangement the balance of the rows are connected to one intermediate source and the columns to a second intermediate source; e.g., voltage sources of one-third and two-thirds that applied to the selected elements. Under these conditions, the power in any one of the balance of the elements in the leakage paths is one-ninth that applied to the selected elements; well below that required to raise these elements to their printing threshold temperature, so that no spurious printing results. This is similar to the arrangement described in U.S. Pat. No. 3,938,136 entitled "Method and Device for Driving a Matrix Type Liquid Crystal Display Element" issued Feb. 10, 1976 and invented by Hideaki Kawakami, which it utilized to reduce crosstalk in a liquid crystal display. While this arrangement minimizes the power applied to each individual leakage element in the thermal array, the overall power may be relatively high since each of these leakage elements has the same voltage applied thereto. This can be of particular concern in a large matrix array as it substantially increases the power requirements of the drive circuitry.